1. Field
The present invention relates to a gradient coil, a magnetic resonance imaging apparatus and a gradient coil manufacturing method. In particular, the invention relates to a gradient coil adapted to form a gradient magnetic field in a main magnetic field direction, a magnetic resonance imaging apparatus including this gradient coil and a method of manufacturing this gradient coil.
2. Description of the Related Art
In a magnetic resonance imaging apparatus (MRI apparatus) for medical use, a main magnetic field is formed with a superconductive magnet or the like, and a Z-gradient coil to form a gradient magnetic field in the main magnetic field direction is employed. Further, a X-gradient coil and a Y-gradient coil adapted to respectively form gradient magnetic fields in X-axis and Y-axis directions orthogonal to the main magnetic field direction are employed.
The present invention relates to the Z-gradient coil among these coils. Therefore, hereinafter, a “gradient coil” will be referred to as the Z-gradient coil in the Z-axis direction.
The magnetic resonance imaging apparatus has a cylindrical bobbin which is substantially similar to a cylindrical space into which a person to be inspected is conveyed. The gradient coil is formed by winding a conducting wire round the bobbin in its circumferential direction by a plurality of turns. In general, the pitches (the intervals among the respective turns of the conducting wire) of respective turns are set irregular. For example, the pitches reach the thinnest states at the center of the axis of the bobbin, become gradually denser from the center of the bobbin axis toward one end thereof, then reach the densest states at a predetermined distance away from the center of the bobbin axis and become gradually thinner after that. Likewise, the conducting wire is wound round the bobbin such that the pitches of respective turns become gradually denser first and then become gradually thinner from the center of the bobbin axis toward the other end thereof.
The conducting wire is wound symmetric relative to the center of the bobbin axis and the direction of a current flowing from the center of the bobbin axis toward one end thereof is reversed relative to the direction of a current flowing from the center of the bobbin axis toward the other end thereof. As a result, there can be formed magnetic fields directed in positive and negative directions with the center of the bobbin axis set as the origin and there is formed a magnetic field which is gradient in the bobbin axis direction as a whole.
The pitches of respective turns of the conducting wire are made irregular in order to increase the linearity of the gradient magnetic field and the positions of respective turns are analytically determined in advance.
As a conducting wire winding method, two methods called “spiral winding” and “lane change winding” are well known as disclosed in JP-A 10-172821.
The spiral winding is a method of literally spirally winding a conducting wire such that the positions of respective turns align with analytically determined positions thereof at predetermined points on the circumference of the bobbin. In the spiral winding, the conducting wire skews in the axial direction on the circumferential surface of the bobbin in one turn and the position of the conducting wire is continuously shifted in the Z-axis direction.
On the other hand, in the lane change winding, the position of the conducting wire is not shifted in the Z-axis direction in one turn, that is, the position is fixed. When the conducting wire is moved from one turn to the next turn, the position of the conducting wire is stepwise shifted (lane-changed) in the Z-axis direction at specific positions on the circumference of the bobbin. In the lane change winding, the position of the conducting wire is discretely shifted in the Z-axis direction.
As disclosed in JP-A 10-172821, in general, the lane change winding is inferior to the spiral winding in workability. The reason therefor lies in that, in the lane change winding, work in a position where the conducting wire is to be lane-changed requires great skill and the workability is reduced particularly in a region where the pitches of respective turns are dense.
To the contrary, the spiral winding is excellent in workability. By the spiral winding, the conducting wire can be wound in a shorter time period than that attained by the lane change winding. As a result, it becomes possible to provide a low-cost gradient coil. JP-A 10-172821 discloses a technique relating to a gradient coil which is superior to a conventional spiral winding method in performance, though the spiral winding which is excellent in workability is adopted, that is, capable of realizing a performance closer to a design value.
Incidentally, the gradient coil for the Z-axis aims to form a gradient magnetic field only in the Z-axis direction and hence it is not preferable to generate magnetic field components in the X-axis and Y-axis directions by this gradient coil. Hereinafter, the magnetic field components generated in the X-axis and Y-axis directions by this gradient coil will be referred to as unnecessary magnetic field components.
In the above mentioned lane change winding, the position of the conducting wire is not shifted in the Z-axis direction in one turn, so that the conducting wire is always orthogonal to the Z-axis. Therefore, the magnetic field induced by a current flowing through the conducting wire is always directed in the Z-axis direction and hence, in principle, the magnetic field component does not generate in either the X-axis direction or the Y-axis direction. That is, it is known that, by the lane change winding, in principle, the unnecessary magnetic field component is not generated, and actually the unnecessary magnetic field component is substantially reduced to zero.
On the other hand, by the spiral winding, the conducting wire skews in the Z-axis direction on the circumferential surface of the bobbin in one turn. Thus, a magnetic field component directed in the X-axis or Y-axis direction, that is, an unnecessary magnetic field component is generated, albeit only slightly.
Recently, the diameter of the cylindrical space of a magnetic resonance imaging apparatus has been more and more increased, and the diameter of the gradient coil and the magnitude of the magnetic field generated by this coil have been also more and more increased accordingly. Likewise, the magnitude of the unnecessary magnetic field component induced by the spiral winding has been increased accordingly. As a result, in the gradient coil adopting the spiral winding, the magnitude of the unnecessary magnetic field component which has scarcely exerted a bad influence so far has been increased to such an extent that cannot be neglected.